PostgreSQL 11 新特性解读: psql 新增 \\gdesc 显示查询结果的列名和类型

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PostgreSQL 11 新特性解读: psql 新增 \\gdesc 显示查询结果的列名和类型

francs.tan 2019-01-08 11:15:17 浏览1102
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PostgreSQL 11 的 psql 新增 gdesc 选项,此选项可以返回查询结果的列名和类型,而不实际执行SQL。

Release 说明

psql
Add psql command gdesc to display the column names and types of the query output (Pavel Stehule)

gdesc 选项说明

gdesc

Shows the description (that is, the column names and data types) of the result of the current query buffer. The query is not actually executed; however, if it contains some type of syntax error, that error will be reported in the normal way.
If the current query buffer is empty, the most recently sent query is described instea

gdesc 只是显示查询结果的列名和类型,并不实际执行SQL,下面演示下。

gdesc 选项演示

数据库中存在一张大表big,结构如下:

[pg11@pghost2 ~]$ psql francs francs
psql (11beta3)
Type "help" for help.

francs=> \d big
                                  Table "francs.big"
  Column   |              Type              | Collation | Nullable |      Default
-----------+--------------------------------+-----------+----------+-------------------
 user_id   | integer                        |           |          |
 user_name | text                           |           |          |
 ctime     | timestamp(6) without time zone |           |          | clock_timestamp()
Indexes:
    "idx_big_ctime" btree (ctime)
    "idx_big_username" btree (user_name)

执行以下查询,如下:

francs=> \timing
Timing is on.

francs=> SELECT count(*),sum(hashtext(user_name)) FROM big;
  count   |      sum
----------+----------------
 30000000 | 11924569894736
(1 row)

Time: 1347.527 ms (00:01.348)

执行时间为 1347 ms 左右。

使用 gdesc 选项查询,如下:

francs=> SELECT count(*),sum(hashtext(user_name)) FROM big \gdesc
 Column |  Type
--------+--------
 count  | bigint
 sum    | bigint
(2 rows)

Time: 0.634 ms

以上返回了查询结果的列和数据类型,执行很快,只需要 0.634 ms,可见没有实际执行SQL。

另一个示例,查询 pg_class 系统表,如下:

francs=> SELECT * FROM pg_class \gdesc
       Column        |     Type
---------------------+--------------
 relname             | name
 relnamespace        | oid
 reltype             | oid
 reloftype           | oid
 relowner            | oid
 relam               | oid
 relfilenode         | oid
 reltablespace       | oid
 relpages            | integer
 reltuples           | real
 relallvisible       | integer
 reltoastrelid       | oid
 relhasindex         | boolean
 relisshared         | boolean
 relpersistence      | "char"
 relkind             | "char"
 relnatts            | smallint
 relchecks           | smallint
 relhasoids          | boolean
 relhasrules         | boolean
 relhastriggers      | boolean
 relhassubclass      | boolean
 relrowsecurity      | boolean
 relforcerowsecurity | boolean
 relispopulated      | boolean
 relreplident        | "char"
 relispartition      | boolean
 relrewrite          | oid
 relfrozenxid        | xid
 relminmxid          | xid
 relacl              | aclitem[]
 reloptions          | text[]
 relpartbound        | pg_node_tree
(33 rows)

这个特性不需要实际执行SQL就能返回查询结果的列和数据类型,在某些特定场景比较有用。

参考

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